The lighthouse isn't really difficult, but there are two glitches that can make it difficult to complete.
When you attempt this there is a glitch that won't let this happen (well for some people anyway). I tried to do this for probably at least ten minutes, and it just won't work. I have seen videos of this on the internet, but it doesn't work for me the same as it does in the video. I also read you can redo the entire tower and it may work the second time around. I did the tower three times and was unable to complete this one activity.
As of September 2017, users are still having issues with these glitches, so it seems unlikely for this to ever be fixed, but hopefully using some of the suggestions in this solution will help unlock it for you.
This next part is a little note on my process and what it took to finally complete this achievement:
I have heard that if you do not skip any cutscenes or pick up collectibles that it is more likely to work, but I did one or both of these things every time and it ended up working on the sixth try.
What I recommend is doing the lighthouse without skipping cutscenes or picking up collectibles and just trying over and over until it works. Hopefully this helps someone.
Little Buburuza recommends clearing your cache if you get caught in the glitches.
Some others recommend attempting multiple times, or restarting console after completing, and going back into the game, which will unlock this for some users.